Do I Need to Fast Before a Blood Test?

If your doctor ordered bloodwork, you might feel confused. You may have heard from friends or relatives that you need to fast — not eat or drink anything — before a blood test.

But is that true for everyone? And is it true for every blood test?

The short answer is no. Here’s what you need to know about blood test fasting.

Do I Need to Fast Before a Blood Test?

You may or may not need to fast before a blood test. The answer depends on what kind of blood test your doctor ordered. That’s why it’s important to be clear on your doctor’s instructions — and ask any questions you may have.

For most blood tests, you don’t need to fast. For instance, if your doctor ordered a complete blood count (CBC), you don’t need to fast. That’s because the food you’ve eaten recently doesn’t affect this test.

Blood Tests That Require Fasting

Some blood tests require that you don’t eat or drink anything (except water) for several hours (or more) before the test. That’s because if you eat and drink normally, those foods get absorbed into your blood.

For example, even a small snack can raise the amount of sugar or fat in your blood. This can affect the results of blood tests that measure blood sugar or fat levels.

If the results of your blood test are incorrect, it can lead to the wrong diagnosis. Also, blood test fasting gives doctors a reliable baseline to compare with future testing.

These are the most common blood tests where your doctor may ask you to fast:

  • Glucose tests. This test, also called a fasting blood sugar test, measures the amount of glucose (sugar) in your blood. It tests for diabetes or prediabetes.
  • Lipid tests. This test measures triglycerides (a type of fat) and cholesterol in your blood. High levels of both can put you at greater risk for heart disease.
  • Iron tests. Doctors use these tests to diagnose iron deficiency anemia.

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How Long Do I Need to Fast?

For most fasting tests, eight to 12 hours of fasting is normal. Your doctor will give you a specific time frame. Your bloodwork appointment will likely be early in the morning, so the majority of your fast will be overnight.

A typical fast is from midnight to 8 a.m., or 8 p.m. to 8 a.m.

Can I Drink Water Before a Fasting Blood Test?

Yes, it’s ok to drink plain water when you’re fasting. Your doctor may actually encourage you to drink water because it keeps your veins full, making it easier to draw blood.

Remember that other drinks are off limits. Coffee and tea (herbal, black, or green) contain plant material that could potentially skew test results.

Coffee and tea also contain caffeine, which is a diuretic and can cause dehydration (even decaf versions contain some caffeine). Being dehydrated may make it harder to draw blood from your vein and may affect test results.

Soda, juice, and alcohol contain high amounts of sugar that can affect your test results.

Other Things to Avoid

When you’re fasting for a blood test, there are some other things you should avoid. The following can stimulate your digestion or otherwise affect your blood test results.

  • Chewing gum, even if sugar-free.
  • Smoking.
  • Vigorous exercise.

Continue to take any prescription medications before your blood test, unless your doctor tells you not to. You may need to avoid medication that’s usually taken with food. Check with your doctor before taking any over-the-counter supplements or vitamins before your blood test.

You can eat and drink normally as soon as your bloodwork appointment is over. Some people like to bring a snack with them so they can eat immediately after the blood test.

What Happens If I Don’t Fast Before a Blood Test?

Everyone makes mistakes. But if you slip up and have your regular morning coffee or breakfast without thinking, tell the person taking your blood. You may have to reschedule your appointment.

Ask with any medical procedure, always check with your doctor if you have any questions about your bloodwork.

Sources
  • NHS, Blood Tests, Link
  • National Heart, Lung, and Blood Institute, Blood Tests, Link
  • National Library of Medicine, How to Prepare for a Lab Test, Link
  • National Library of Medicine, Fasting for a Blood Test, Link
  • National Library of Medicine, Iron Tests, Link
  • American Academy of Family Physicians, Complete Blood Count (CBC), Link
  • CDC, Diabetes Tests, Link

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